I had already upgraded 3 of my 4 lines to iPhone 6s and 6s Plus phones in the last few months but my personal line wasn't due for an upgrade until April 2016 so I did the early upgrade for my line to get a $200 discount because I do plan to keep my unlimited account for at least another 2 years. My phone should be here tomorrow. If you plan to keep your unlimited plan or the old grandfathered subsidized plans you should considering upgrading if you can before 1/8 because after that date you are either paying full price up front or paying extra per month for device financing (either through apple or the next program).
In 2 years when these contracts are up I'll have to consider my options and maybe by then the mobile share plans will make more sense (right now my costs would go up switching to those plans without even adding any device payments). I do have a generous FAN discount on my plan that would only apply to the share portion of the mobile share plans but currently applies to my entire grandfathered plans (voice, text, data, etc.) which is why mobile share is quite a bit more money for me.